This invention relates to electrical power assisted steering systems of the kind in which an electrical motor is adapted to apply an assistance torque to a steering component such as a steering column so as to reduce the driver effort required to control the vehicle.
In a simple electric power assisted steering system a torque sensor is provided which is arranged so that the level of torque in a steering column is measured. From this measurement a controller calculates the value of a torque demand signal that is indicative of the torque that is to be generated by an electric motor attached to the steering column. The motor applies a torque to the column of the same sense as that demanded by the driver and thus reduces the effort needed to turn the wheel.
A problem with this simple arrangement occurs in certain driving manoeuvres that excite a vehicle yaw mode transient response—leading to so-called “fish-tailing” of the vehicle. These manoeuvres are typically the result of “unsupported” driver actions on the handwheel such as rotational “flicks” where the driver applies a rapid handwheel angle change but does not follow it through with any substantial applied torque or perhaps releases the handwheel after initiating a rapid turn.
In such circumstances it is desirable that the handwheel returns to the central “straight-ahead” position quickly and with a minimum amount of overshoot or oscillation. In general, however, geometric and inertial effects of the steering system contribute to a free mode yaw response that is lightly damped and quite oscillatory—particularly at high vehicle speeds.
It is known in the art to overcome this problem by including a torque damping component within the torque demand signal that is used to drive the motor. This torque damping component in some sense mimics the mechanical phenomenon of viscous friction that is present in a traditional hydraulic system.
The damping component generally increases in magnitude as a function of steering column velocity from zero torque at zero rotational speed to a maximum at some arbitrary maximum speed. This can be achieved by applying a scaling factor (sometimes called a boost curve) to the column velocity. In effect, as the torque damping component increases the torque output by the motor decreases, and hence the amount of assistance. This gives increased damping and hence stability at high vehicle speeds.
It is further known to provide an additional damping component that is also a function of the torque carried by the column. This can be achieved by applying various scaling factors to the column velocity dependent damping terms. The damping component will typically be reduced at low torques compared to the magnitude of the damping component at high torques. Thus, in hands free manoeuvres where no torque is present in the column the damping will be relatively high and yet be lower during hands on manoeuvres in which torque is generally present in the column.
It has been appreciated by the applicant that it is possible to improve on the arrangements of the prior art, especially in providing an improved feel to the steering as perceived by a driver. Although not in any way intended to be limiting, the improved feel can be considered to constitute a feel that more closely mimics the feel of a traditional hydraulic steering system.